Long Term Evolution (LTE) is a technology developed by the 3rd Generation Partnership Project (3GPP) for mobile broadband wireless communication. In LTE, transmissions from the base stations (referred to as Evolved Node Bs (eNBs)) to user terminals (referred to as user equipment (UE)) are sent using orthogonal frequency division multiplexing (OFDM). In OFDM systems, such as LTE, a wideband carrier is divided into multiple subcarriers. The basic unit of transmission in LTE is a resource block which, for most configurations, comprises twelve 15 kHz subcarriers in the frequency domain and one 0.5 millisecond slot (seven OFDM symbols) in the time domain.
In order to demodulate transmissions on the downlink, a user terminal first has to synchronize in time and frequency with the base station. When the user terminal initially connects to the cell, it performs a cell search operation. During the cell search operation, the user terminal performs coarse time and frequency synchronization with the base station and determines the cell identity for the cell served by the base station. To assist the user terminal in the cell search procedure, the base station transmits two synchronization signals on the downlink: the primary synchronization signal (PSS) and the second synchronization signal (SSS). The PSS and SSS span only the six resource blocks in the center of the transmitted signal. The possible sequences representing different cell IDs are known a priori to the user terminal, and the user terminal finds the cell ID by correlating the transmitted signal with each of the candidate sequences. After acquiring coarse time and frequency synchronization, during the cell search procedure, the user terminal reads the system information from the broadcast channel. The system information includes, among other things, information about the uplink and downlink cell bandwidth and the common reference symbol (CRS) configuration. The user terminal uses the CRS to demodulate information received on the downlink. The CRS is also used to maintain time and frequency synchronization with the base station.
The CRS spans the entire bandwidth of the signal being transmitted by the base station and occurs in four or six OFDM symbols in every subframe (one millisecond). Therefore, the CRS is useful for both time and frequency synchronization. However, the CRS may lead to energy inefficient transmission because the CRS is transmitted in every subframe, even when no data is being transmitted. Further, the interference between CRS signals from different cells, and the limitation on the number of transmission modes for control channels that use the CRS for channel estimation, limit the capacity for control channels in LTE. Therefore, it would be desirable to reduce or eliminate reliance on CRS for time and frequency synchronization.